1. FIELD OF THE INVENTION
The present invention relates to an apparatus that controls an engine by varying the valve timing of at least either one of an intake valve and an exhaust valve while adjusting the amount of intake air supplied to the engine.
2. DESCRIPTION OF THE RELATED ART
In a typical engine, an intake passage and an exhaust passage, which are communicated with a combustion chamber, are selectively opened and closed by an intake valve and an exhuast valve, respectively. The intake valve is driven by an intake camshaft while the exhaust valve is driven by an exhuast camshaft. The intake and exhaust camshafts rotate synchronously with a crankshaft. Accordingly, the opening and closing timing of the valves, or the valve timing, is determined by the rotational phase of the crankshaft.
There are various types of apparatus that change the valve timing of the intake and exhaust valves in accordance with the operating state of the engine. A typical apparatus is provided with a variable valve timing (VVT) mechanism for changing the valve timing and a computer for controlling the VVT mechanism. The computer controls the VVT mechanism in accordance with the operating state of the engine to vary the valve timing to a target timing. Furthermore, when the valve timing is varied, the period of time during which both intake and exhaust valves are opened, that is, the valve overlap, is also changed.
Changes in the valve timing and the valve overlap optimizes the amount of air-fuel mixture drawn into the combustion chamber, the amount of exhaust gas discharged from the combustion chamber, the discharge timing of the exhaust gas, and the amount of residual exhaust gas in the combustion chamber, or the amount of internal gas recirculation (EGR). This increases engine power and improves fuel emissions and fuel efficiency.
However, when the VVT mechanism fails to function, the actual valve timing becomes different from the target valve timing. This may result in various problems.
For example, if the timing mechanism fails when the valve overlap is long, the increase in the amount of internal EGR decreases the amount of air included in the air-fuel mixture that is sent to the combustion chamber. This causes the amount of air drawn into the combustion chamber to become insufficient and hinders normal combustion. As a result, the fuel efficiency becomes low and the engine emissions becomes undesirable.
An insufficient amount of air may cause the engine to stall especially if the load of the engine is low such as when the engine is idling. The amount of fuel drawn into the combustion chamber for combustion is small when the engine load is low. Thus, in such state, combustion tends to become unstable. An increase in the amount of internal EGR further amplifies the unstable combustion.
A control apparatus that prevents such engine stalls has been proposed in the prior art. As shown in FIG. 15, the control apparatus is applied to an engine 200, which has a crankshaft 201, an intake camshaft 202 for driving a suction valve (not shown), and an exhaust camshaft 203 for driving an exhaust valve (not shown). Pulleys 204, 205, 206 are provided on the ends of the shafts 201, 202, 203, respectively, and connected to one another by a timing belt 207.
The control apparatus includes a VVT mechanism 211, which shifts the valve timing of the intake valve, and a computer 212, which controls the VVT mechanism 211. The VVT mechanism 211 is provided with a control plate 208, which rotates relatively to an intake camshaft 202, an actuator 210, which rotates the control plate 208, and a link 209, which connects the actuator 210 and the control plate 208. The actuator 210 rotates the control plate 208 about the intake camshaft 202 with the link 209.
When the engine 200 is idling, the computer 212 controls the actuator 210 to rotate the rotate the control plate 208 counterclockwise to a position shown in FIG. 16(a). This position is hereafter referred to as the idling position. The rotation of the control plate 208 retards the valve timing of the intake valve and causes the valve overlap to become shorter.
When the load of the engine 200 is high, the computer 212 controls the actuator 210 to rotate the control plate 208 clockwise to a position shown in FIG. 16(b). This position is hereafter referred to as the high load position. The rotation of the control plate 208 advances the valve timing of the intake valve and causes the valve overlap to become longer.
Accordingly, the control apparatus enables the valve timing of the intake valve and the valve overlap to be shifted between two conditions.
Furthermore, as shown in FIG. 15, the engine 200 is provided with an intake manifold 213. A surge tank 214 and an intake pipe 215 are further provided at the upstream side of the intake manifold 214. A throttle valve 216 is arranged in the intake pipe 215. The throttle valve 216 adjusts the amount of air that is drawn into the combustion chamber (not shown) by way of the intake pipe 215, the surge tank 214, and the intake manifold 213.
The intake pipe 215 is provided with a bypass 217. The bypass 217 connects the upstream side of the throttle valve 216 to the downstream side of the throttle valve 216. An idle speed control valve (ISCV) 218 is arranged in the bypass 217. When the throttle valve 216 is completely closed during idling of the engine 200, the ISCV 218 adjusts the speed of the engine 200 by adjusting the amount of air flowing through the bypass 217.
When the engine 200 shifts to an accelerating state from an idling state, the computer 212 controls the ISCV 218 and increases the opening of the ISCV 218. Thus, engine stalls caused by an insufficient amount of intake air is prevented even if the VVT mechanism 211 fails to function when the engine 200 returns to an idling state.
The amount of air tends to become insufficient due to an increase in the amount of internal EGR if the VVT mechanism 211 fails to function when the control plate 208 is located at the high load position. Therefore, to positively prevent engine stalls, it is necessary to set the increase amount of the opening of the ISCV 218 under the assumption that the VVT mechanism 211 fails to function when the control plate 218 is located at the high load position.
However, if the increase amount of the opening of the ISCV 218 is determined assuming that the VVT mechanism 211 fails when the control plate 208 is located at the high load position, excessive air is drawn into the combustion chamber by way of the bypass 217 when the engine 200 shifts from an idling state to an accelerating state. This may suddenly increase the output torque of the engine 200 and degrade driveability. This may also decrease the effectiveness of the engine brake.
Engine stalls may be prevented by detecting abnormalities of the VVT mechanism 211 and increasing the opening of the ISCV 218 when detecting abnormalities. If the VVT mechanism 211 functions normally, excessive air is not drawn into the engine 200 when the engine 200 shifts from an idling state to an accelerating state.
However, a certain amount of time is necessary to detect abnormalities of the VVT mechanism 211. This delays the response of the VVT mechanism 211. Thus, the engine 200 may stall when shifted from an idling state to an accelerating state during the period between when an abnormality actually occurs and when the abnormality is detected.